High-power ion and electron sources in cascade arrangement



April 18,- 1967 HIGH-POWER ION AND ELECTRON SOURCES IN CASCADEARRANGEMENT Filed June 14,. 1963 H. FRCHLICH 3,315,125

United States Patent Office 3,L'-ll5,l25 Patented Apr. 18, 19673,315,125 HIGH-POWER ION AND ELECTRON SOURCEfi lN tCAS CADE ARRANGEMENTHeinz Friihlich, Nurnberg, Germany, assignor to Siemens- SchuckertwerlreAlrtiengescllschaft, Berlin-Siemensstadt,

Germany, a corporation of Germany Filed June 14, 1963, fier. No. 287,817priority, application Germany, Nov. 20, 1962,

8 82,505 8 Claims. (Cl. 315-111) My invention relates to high-power ionand electron sources and more particularly ion and electron sourcesbased upon the principle of the so-called du-oplasrnatron sources knownfrom German Patent 1,059,581.

Such high-power ion and electron sources are capable of producingelectron streams of up to about amperes, and when fed with hydrogen, canproduce ion streams of up to about 1 ampere in continuous operation.However, for many technical uses, in high-vacuum melting ovens formetals with high melting points for example a greater electron output isdesired. Thus, there are melting ovens of up to 600 kilowatts powerrating which require three sources with three acceleration and vacuumsystems for the normally employed voltages of 20 kv. On the other hand,the production of ions by these sources is insufiicient for specialpurposes, such as ionic drives for space travel. In order to producemore intense ion streams, it has been suggested that straight orring-shaped emission slots be made in the duoplasmatron. Such slotteddesigns, however, are usually not suitable for technical use as anelectron source. It is diflicult, furthermore, to construct cathodes ofsuitable shape, efiiciency, useful life span, and service reliabilityfor the expanded arc discharge occurring in such sources, since arccurrents of about 100 amperes have to be kept under control.

An object of my invention is to provide a high-power ion and electronsource assembly employing the principle of duoplasmatrons to producevery intense ion streams without requiring specially shaped cathodes.

Another object of the invention is to provide a source assembly with atleast two stages in which all but the first stage require no cathodestructure at all.

A further object of the invention is to provide an assembly of theabove-described character which because of its ability to produce ionstreams of relatively great intensity, makes new fields of applicationwhere high discharge currents are desired or required, accessible foruse of sources on the duoplasmatron principle.

With the above and other related objects in view, and

in accordance with my invention, I provide at least two componentsources arranged in cascade, and have the electrons emitted from thepreceding source constitute a virtual cathode of the following source,the discharge currents of the component sources increasing by the use ofvolume ionization from stage to stage. Only one cathode of normal sizefor an emission current in the order of 10 amperes is then required inthe first stage of the cascade, while a current many times greater canbe drawn from the last stage of the cascade. Other features which areconsidered as characteristic for the invention are set forth in theappended claims. The invention, however, both as to its construction andmethod of operation, together with additional objects and advantagesthereof, will be best understood from the following description ofspecific embodiments when read in connection with the accompanyingdrawings, in which:

FIG. 1 is explanatory and shows diagrammatically a sectional view of theupper part of a duoplasmatron source; and

Claims FIG. 2 is a two-stage embodiment of the invention shown partlydiagrammatically and partly in section.

Referring to the drawings and first particularly to FIG. 1 there isshown an electron beam 1 which is emitted from a source S through anaperture 1a in an anode 3 with an energy of up to 30 electron volts. Adischarge chamber 2a is connected to this source S and has a gaspressure approximately equal to the pressure in the source. Theelectrons ll .emitted by the source serve as a virtual cathode for thedischarge 2 in the discharge chamber 2a, which is enclosed by a pullinganode 4 at one end and a cylindrical wall 5. A direct current source 6energizes the discharge.

It is essential to the invention that the discharge 2, within the properrange of operating voltage between the extraction anode 4 and the sourceanode 3, is of the type operating without a cathode spot (burning spot).Since the electrons 1 are emitted with considerable energy from thesource S, there is also no necessity for formation of a cathode drop. Asa result, in the aforementioned voltage range, the discharge burns witha positive voltage characteristic so that no ballast resistance isnecessary for limiting the current in the circuit. The ions impinging onthe source anode 3 distribute themselves on the entire surface thereof.The above-described operating conditions constitute the stableduoplasmatron operation.

It has been found that, by utilizing the volume ionization in thedischarge 2, a discharge current of double or treble the amount of theemission current of the virtual cathode ll can be drawn in the discharge2 under stable operating conditions, so that the discharge current isequal to or larger than the arc current in the source. This elfect is tobe attributed to the development of electrons by ionization processes.Only when a critical value of the extraction voltage is exceeded doesthe discharge change to a true are discharge with a negativecharacteristic.

This phenomenon can be utilized by arranging a plurality of sources incascade one behind the other, so that each source acts as a virtualcathode (in the electronoptical sense) for the following source. The arecurrent is thereby multiplied without requiring a correspondingly largeincandescent cathode.

As can be seen in FIG. 2, the first or preceding stage includes acathode flange 7 of non-magnetic material, for example refined orstainless steel, on which is mounted an insulating bushing 8 throughwhich extend cathode lead elements 9 that are electrically connected toa cathode It). An intermediate electrode 11 insulated by a ring 14 froman outer covering 12 and from an anode plate 13a that is formed with anemission opening 13, is cooled by cooling pipes 15 and is provided witha canal 16 which cooperates with the anode plate to produce a magneticpole-shoe lens for guiding the plasma through the emission aperture 13.The lens 13a, 16 is magnetically energized by a coil 17 which isenergized from a non-illustrated direct current source. A low-voltageincandescent discharge takes place between the cathode 10 and the anodeplate 13a when a gas pressure of at least 10- mm. Hg is gas feed pipe13. The gas flows through the emission aperture 13, and, in theillustrated embodiment of FIG. 2, ultimately flows out of any emissionopening 30. The gas feed pipe or inlet 18 may, however, also beconnected to the upper stage of the device or any added intermediatestage. The low-voltage discharge is fed across a stabilizing resistor bya current source 19, which can have a potential of between 200 to 300volts. The shielded construction shown in the drawing basicallycorresponds to the construction disclosed in the previously 3 describedGerman Patent 1,059,581 in which I am named as the inventor.

The intermediate electrode 11 that is shown in FIG. 2 as being connectedto the casing 12 through the switch 21 and the resistor 22, is biased tosuch a negative potential with respect to the casing that the electronstream from the intermediate electrode is equal to the ion stream. Thecombined arc stream flows, in this case, through the emission opening 13from the cathode it) with a slight percentage loss and with a regulatedpotential of about 30 volts. Investigations have shown that doublelayers (potential jumps) arise in the plasma, that is in the canal 16,which accelerate the electrons to this velocity in the direction of theemission aperture 13.

Due to the constriction or pinching of the plasma by the magneticpole-shoe lens, i.e. by the spiralling of the electrons in theinhomogeneous magnetic field of the lens, an intense ionization of thegases occurs between the opening of the canal to and the emissionaperture 13 which leads to a positive plasma potential with respect tothe anode. A positive potential hill is therefore formed, reaching itsmaximum directly over the canal opening from which the ions areaccelerated in the direction of the canal and in the direction of theemission aperture. For electrons, instead of a positive potential hillthere is a potential trough, and in this trough the electrons that areformed by ionization action in this range oscillate and furtherintensify the ionization.

If the intermediate electrode 11 is connected by the switch 21 to thenegative pole of the generator 19 through a resistor 23, theintermediate electrode 11 is given a strong negative potential withrespect to the anode 13a. Ionic wall losses in the canal 16 are therebyincreased and theion stream from the positive potential hill to thecanal becomes larger. The plasma gradient thereby becomes greater andthe electrons are accelerated even more strongly toward the emissionopening 13. The ions formed by ionization action can then also beaccelerated and emitted so that it is possible to draw an electroncurrent even from the first stage that is about 30 to 50% greater thanthe cathode emission. The intermediate electrode thus has a potentialthat is somewhat similar to the cathode potential.

The anode plate 13a is cooled with water or a similar suitable coolantcirculated through a bore 24 formed therein. With an emission aperture13 of a diameter of 1.3 to 1.4 millimeters in the first stage, anelectron current of about amperes can be produced.

The electron current then enters the second stage which is separatedfrom the first stage by an insulating ring 25. Between the casing 12 ofthe first stage and the casing 26 of the second stage, there is anelectromotive force supplied by the generator 27, no ballast resistorbeing required in the circuit. In further respects, generally, theconstruction elements of the second stage correspond to those of thefirst stage and require no additional explana tion.

It is essential, however, that the discharge in the second stage diifersfrom the discharge in the first stage in that it burns with a positivevoltage characteristic. The voltage of the generator 27 can be soadjusted that with the aid of an intermediate electrode 28 held at anegative potential (when switch 29 is in the lower position), anelectron current is emitted through the opening 30 that is two to threetimes larger than the emission current out of the opening 13. An ionreverse current strikes the ion plate 13a of the first stage along itsentire cooled surface. This is true generally for the anodes of all thestages just so long as the arc in each has the described characteristic.

The geometry of the pole-shoe lens in the second and following stagescan be so constructed that the density of the stream in the respectiveemission opening may have a predetermined value, for example may remainconstant. The larger the emission opening is, the more easily can it beaccorded a canal-like characteristic because the wall losses ofelectrons become relatively small and therefore permit better heatremoval. The size of the beam cross section is determined by suitablechoice of the number of ampere windings of the energizing coil.

In contrast to the normal single-stage duoplasmatron in which the plasmacross section and the cross section of the emission opening must conformwith each other as much as possible to avoid neutral gas losses, in thepresent invention, with the exception of the upper stage, as shown inFIG. 2, the cross section of the emission openings is larger than theplasma beam cross section. The heat load of the respective anodes isthereby decreased, and adjustment of. the gas pressure in the individualstage is facilitated.

The inner walls of the canals of the pole-shoe lenses that are locatedin the intermediate electrodes can be provided with thin metal cylindersfor heat-insulating the canal walls, which, due to ion impact, areconsequently heated to such a high temperature that they emit electrons.These electrons contribute to the increase of the combined emission ofthe particular stage. In FIG. 2 there is schematically shown a metalcylinder 31 with a heat insulator 32.

While the invention has been illustrated and described as embodied in aparticular high-power ion and electron source assembly, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing from the spirit ofthe present invention and within the scope and range of equivalents ofthe following claims.

I claim:

1. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and at least onesucceeding cathodeless stage, otherwise substantially similar to thepreceding stage, said preceding and succeeding stages being arranged incascade, the electrons discharged by the preceding stage being receivedin the chamber defined by the intermediate electrode of the cathode-lesssucceeding stage and forming a virtual cathode for the same, thedischarge currents of said stages being increased from stage to stagethrough volume ionization of the gas.

2. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and at least onesucceeding cathode-less stage, otherwise substantially similar to thepreceding stage, said preceding and succeeding stages being arranged incascade, the electrons discharged by the preceding stage being receivedin the chamber defined by the intermediate electrode of the cathode-lesssucceeding stage and forming a virtual cathode for the same, the anodeof the succeeding stage being biased with respect to the virtual cathodeto attract the electrons thereof, and means for adjusting the attractingvoltage of the anode of the suc ceeding stage so that a dischargecurrent with a positive voltage characteristic and without cathodevoltage drop is maintained, the discharge currents of said stages beingincreased from stage to stage through volume ionization of the gas.

3. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and at least onesucceeding cathode-less stage, otherwise substantially similar to thepreceding stage, said preceding and succeeding stages being arranged incascade, first energized external circuit means connecting the anode andcathode of the preceding stage, and second energized external circuitmeans interconnecting the anode of the preceding and succeeding stages,only said first external circuit means including a ballast resistance,the electrons discharged by the preceding stage being received in thechamber defined by the intermediate electrode of the cathode-lesssucceeding stage and forming a virtual cathode for the same, thedischarge currents of said stages being increased from stage to stagethrough volume ionization of the gas.

4. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and at least onesucceeding cathode-less stage, otherwise substantially similar to thepreceding stage, said preceding and succeeding stages being arranged incascade, voltage means connecting the anode and the intermediateelectrode of each stage respectively, and biasing said intermediateelectrode strongly negative with respect to its anode, the electronsdischarged by the preceding stage being received in the chamber definedby the intermediate electrode of the cathode-less succeeding stage andforming a virtual cathode for the same, the discharge currents of saidstages being increased from stage to stage through volume ionization ofthe gas.

5. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and at least onesucceeding cathode-less stage, otherwise substantially similar to thepreceding stage, said preceding and succeeding stages being arranged incascade, voltage means connecting the anode and the intermediateelectrode of each stage, respectively, and biasing said intermediateelectrode strongly negative with respect to its anode, and means foradjusting the bias between said intermediate electro-de and said anode,respectively, the electrons discharged by the preceding stage beingreceived in the chamber defined by the intermediate electrode of thecathode-less succeeding stage and forming a virtual cathode for thesame, the discharge currents of said stages being increased from stageto stage through volume ionization of the gas.

6. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode andinter-mediate electrode, said intermediate electrode defining a chamberin which an ionizable gas is received, and cooperating with said anodeand cathode, when respectively energized, for ionizing the gas and fordischarging electrons through an aperture formed in said anode, and atleast one succeeding cathodeless stage, otherwise substantially similarto the preceding stage, said preceding and succeeding stages beingarranged in cascade, voltage means connecting the anode and theintermediate electrode of each stage, respectively, and biasing saidintermediate electrode strongly negative with respect to its anode, andswitching means for selectively disconnecting said voltage means fromsaid anode and intermediate electrode, respectively, and connecting saidanode and intermediate electrode to one another across a ballastresistance, the electrons discharged by the preceding stage beingreceived in the chamber defined by the intermediate electrode of thecathode-less succeeding stage and forming a virtual cathode for thesame, the discharge currents of said stages being increased from stageto stage through volume ionization of the gas.

7. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is receive-d, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons through an aperture formed in said anode, and means forcombining at least part of the ionized gas and electrons in saidpreceding stage into a plasma beam and guiding said beam toward theaperture in said anode, and at least one succeeding cathodedess stage,otherwise substantially similar to the preceding stage, said precedingand succeeding stages being arranged in cascade, the electronsdischarged by the preceding stage being received in the chamber definedby the intermediate electrode of the cathode-less succeeding stage, saidelectrons forming a virtual cathode for the same, the discharge currentsof said stages being increased from stage to stage through volumeionization of the gas, the cross section of the aperture formed in theanode of each stage being larger than the cross section of the plasmabeam in the respective stage except for the last stage of the cascade.

8. A high-power ion and electron source, comprising in combination, apreceding stage including an energizable anode, cathode and intermediateelectrode, said intermediate electrode defining a chamber in which anionizable gas is received, and cooperating with said anode and cathode,when respectively energized, for ionizing the gas and for dischargingelectrons, said intermediate electrode having an end wall formed with acanal communicating with said chamber and being in registry with anaperture formed in said anode, said canal being provided with aperipheral heat-insulating thin metal layer, and at least one succeedingcathode-less stage, otherwise substantially similar to the precedingstage, said preceding and succeeding stages being arranged in cascade,the electrons discharged by the preceding stage being received in thechamber defined by the intermediate electrode of the cathode-lesssucceeding stage and forming a virtual cathode for the same, thedischarge currents of said stages being increased from stage to stagethrough volume ionization of the gas.

References Cited by the Examiner UNITED STATES PATENTS 2,975,277 3/1961Von Ardenne 315-111 X 3,033,984 5/1962 Fisher et al 250-833 3,137,8016/1964 Brooks et a1 313-161 X JAMES W. LAWRENCE, Primary Examiner. S. A.SCHNEEBERGER, Assistant Examiner.

2. A HIGH-POWER ION AND ELECTRON SOURCE, COMPRISING IN COMBINATION, APRECEDING STAGE INCLUDING AN ENERGIZABLE ANODE, CATHODE AND INTERMEDIATEELECTRODE, SAID INTERMEDIATE ELECTRODE DEFINING A CHAMBER IN WHICH ANIONIZABLE GAS IS RECEIVED, AND COOPERATING WITH SAID ANODE AND CATHODE,WHEN RESPECTIVELY ENERGIZED, FOR IONIZING THE GAS AND FOR DISCHARGINGELECTRONS THROUGH AN APERTURE FORMED IN SAID ANODE, AND AT LEAST ONESUCCEEDING CATHODE-LESS STAGE, OTHERWISE SUBSTANTIALLY SIMILAR TO THEPRECEDING STAGE, SAID PRECEDING AND SUCCEEDING STAGES BEING ARRANGED INCASCADE, THE ELECTRONS DISCHARGED BY THE PRECEDING STAGE BEING RECEIVEDIN THE CHAMBER DEFINED BY THE INTERMEDIATE ELECTRODE OF THE CATHODE-LESSSUCCEEDING STAGE AND FORMING A VIRTUAL CATHODE FOR THE SAME, THE ANODEOF THE SUCCEEDING STAGE BEING BIASED WITH RESPECT TO THE VIRTUAL CATHODETO ATTRACT THE ELECTRONS THEREOF, AND MEANS FOR ADJUSTING THE ATTRACTINGVOLTAGE OF THE ANODE OF THE SUCCEEDING STAGE SO THAT A DISCHARGE CURRENTWITH A POSITIVE VOLTAGE CHARACTERISTIC AND WITHOUT CATHODE VOLTAGE DROPIS MAINTAINED, THE DISCHARGE CURRENTS OF SAID STAGES BEING INCREASEDFROM STAGE TO STAGE THROUGH VOLUME IONIZATION OF THE GAS.